1. Field of the Invention
The invention pertains to counterbalance mechanisms, more specifically to a window sash counterbalance having a torsional spring that is torqued by a stiff spiral rod that drives a follower attached to one end of the spring as the spiral rod is drawn through the follower by movement of the window sash.
2. Description of the Prior Art
Counterbalancing mechanisms provide the user with easy and safe product operation by compensating for a product's weight when the item must be moved vertically.
Counterweight has been used comprising a lead weight suspended from a rope which passes over a pulley at the top of the window frame and is attached to the edge of the window sash whereat an upward pull is made upon the sash.
Overhead garage doors are often counterbalanced by an extension spring which runs horizontally along the rails of the garage door or a torsional spring mounted above the door.
A counterbalance mechanism, described in U.S. Pat. No. 2,817,872, patented Dec. 31, 1957 by E. E. Foster, has a self-coiling coil ribbon spring under constant tension attached to the window frame or head jamb and the window sash, which is wound and unwound by vertical movement of the sash.
U.S. Pat. No. 1,864,745 patented Jun. 28, 1932 by A. Larson describes a window sash balance in which a tubular coil spring is attached by one end of the spring to the upper end of the window frame and is fixed against rotation at the attachment.
A cap having a cylindrical opening with an internal constriction is fixedly mounted to the spring on the other end of the spring which is not attached to the window frame or the window sash.
An elongated spiral rod has a convoluted longitudinal groove that provides a pair of longitudinal ridges that define the pitch of the spiral. One end of the rod is attached to the sash, fixed against rotation at the attachment. The other end of the rod extends through the constriction in the cap and axially into the spring.
The balance is installed on the lower sash and frame with the spiral portion of the rod disposed inwardly of the coil and with the lower sash raised. When the sash is lowered to the closed position, the coil spring is twisted or torqued by the cap which is forced by the spiral to rotate as the spiral moves in extension from the spring through the internal constriction. When the lower sash is raised, the coil spring is revolved around the spiral by the cap in the opposite direction thus releasing the torsion acquired when the sash was lowered.
The spring may be pretorqued by detaching and twisting it at the end attached to the frame.
The pitch of the convoluted groove increases gradually from one end of the rod to the other end of the rod, the pitch being less at the end attached to the sash. When the sash is in the closed position the spiral is retracted out of the coil and the coil is wound to maximum tension. The coil revolves relatively slowly at the higher pitch end of the rod. The increased pitch prevents the highly torqued spring from drawing the window open by drawing the rod into the spring.
The gradually decreasing pitch provided in the spiral toward the end attached to the sash facilitates rotary movement of the coil as the sash is moved into the open position and torque in the coil is consequently decreased. The rotary movement of the coil about the spiral is relatively fast at first as the sash is moved into the closed position from the extended position and the movement of the coil is relatively slow as the spiral reaches the extended position.
U.S. Pat. No. 5,267,416 patented Dec. 7, 1993 by D. Davis describes a torsion spring window balance having a torsion spring in a rigid tube. The first end of the spring is fixedly attached to the first end of the tube. A nut is journalled within the second end of the tube and is connected to the second end of the torsion spring for rotating the second end of the spring. A spiral rod threadably engages the nut and extends into the spring.
The first end of the tube is attached to the window frame. The end of the rod that is outside the spring is attached to the sash shoe to communicate in the form of lifting force on the sash, the tensive force between the spiral rod and the tube transformed by the nut from the torque produced between the ends of the torsion spring.
The pitch of the spiral rod is varied according to an algebraic quadratic relationship. It provides a lifting force along about the lower 80% of sash vertical travel and about a 5% higher lifting force for the remaining upper travel or raised positioning of the sash.